(Not Applicable)
1. Technical Field
This invention concerns cellular communications, and more particularly RF repeater systems.
2. Description of the Related Art
Conventional wireless cellular communications systems have a common architecture in which one or more defined cell sites are formed by the placement of one or more base transceiver stations within a geographic area. A cell site is typically depicted as a hexagonal area in which a transceiver is located such that a radio communication link can be established between the cellular system and a plurality of mobile stations within the defined geographic area for the cell.
In order to extend the coverage of conventional base transceiver station (BTS) systems over a larger geographical area, cell service providers have found it useful to employ translating repeaters. In the uplink direction, signals transmitted by a mobile station (mobile transceiver unit) located in a remote cell are received at the repeater, translated to a different carrier frequency, and then transmitted to the host BTS. Likewise, in the downlink direction, signals transmitted by the host BTS are received by the repeater, translated to a different carrier frequency, and then transmitted to mobile stations. The RF carrier link between the repeater and the BTS is known as the xe2x80x9cbackhaul channel,xe2x80x9d hereinafter, backhaul channel, and the carrier frequency on which the backhaul channel is operated is called the xe2x80x9cbackhaul frequency.xe2x80x9d
Some translating repeaters, such as the AirSite(copyright) repeater system offered by AirNet Communications Corporation of Melbourne, Fla. advantageously make use of existing in-band RF carrier frequencies to backhaul cellular communications traffic. As used herein, the term xe2x80x9cin-bandxe2x80x9d refers to carrier frequencies that are within the frequency spectrum allocation assigned to the service provider for providing cellular communications services to mobile subscribers. Use of in-band radio frequency channels to backhaul cellular communications traffic from remote repeater sites is highly advantageous as it eliminates costly wireline T1 or microwave connections.
While use of in-band radio frequency channels to backhaul cellular communications traffic has distinct advantages, it also has some drawbacks. For example, in conventional wireless translating repeaters, a full duplex backhaul channel requires a pair of corresponding uplink and downlink backhaul RF carrier frequencies. Use of such in-band channels for providing a backhaul link necessarily reduces the number of channels available to a service provider on which to communicate with mobile subscribers. As mobile subscriber traffic increases, additional RF carrier channels must be allocated for the backhaul function. In sectorized systems, a corresponding backhaul transceiver must be provided for each sector transceiver operating for a given site.
In conventional translating repeaters, the digitally coded voice data as received from the mobile units is automatically re-transmitted to the base station without modification of the encoded data. In systems conforming to the GSM standard/specification, voice data is always transmitted using Gaussian Minimum Shift Keying (GMSK) modulation scheme along with both frequency division multiple access (FDMA) and time division multiple access (TDMA) schemes. In accordance with the GSM standard/specification, information is carried in frames in each uplink and downlink carrier frequency channel. Each GSM frame is divided into time slices called timeslots, with each frame having 8 timeslots.
While the foregoing GSM system ensures that at least eight mobile units can be assigned to a single carrier frequency, it still requires a dedicated pair of uplink and downlink carrier frequencies for each backhaul channel. This creates a problem since the number of carrier frequencies or channels assigned to each service provider is limited. This is particularly true when there are a number of translator base stations, with each requiring its own assigned set of backhaul carrier frequencies.
Another problem with conventional translating repeaters is that they do not perform complete radio frequency RF demodulation and modulation functions. Instead, they downconvert the RF signal to an intermediate frequency (IF) and then up-convert the signal to the translated RF carrier frequency. Consequently, bit error rate (BER) performance is not improved, and can actually be made worse. A further problem is that, in a conventional repeater, the GSM slot/frame timing information for the uplink signal is derived as an offset to the uplink signal.
Another disadvantage of conventional repeaters is that power level control of the downlink and uplink signals is performed using RF envelope detection, signal delays, microcontroller look-up tables, and digitally-controlled RF step attenuators. This analog circuitry used to accomplish these functions is susceptible to variations in temperature since the equipment housing the repeater stations are remotely located in small outdoor huts. The equipment must therefore be designed to make the necessary compensation and adjustments to ensure proper functionality.
Yet another disadvantage of conventional repeaters is that diversity switching of multiple uplink signals is performed using RF envelope detection, analog signal delay, microcontroller processing, and a digitally controlled RF switch. This circuitry is also susceptible to variations in temperature and requires careful compensation and adjustment for proper functionality. Similarly, with conventional wireless repeater systems, in-band control of the repeater and repeater alarm notification is difficult to perform since control bits must be demodulated from the downlink path and alarm/status bits must be modulated onto the uplink path.
The instant invention discloses a method for supporting increased capacity on a backhaul communication link carrier frequency in a wireless communication system having a base station located within a home cell, and a plurality of substantially adjacent cells each having a repeater station located therein. The method comprises receiving at one of the repeater stations, a first RF signal having a first modulation scheme from a mobile transceiver unit on an uplink channel. The first RF signal from the mobile transceiver unit is demodulated at the repeater station to obtain a digital data stream. The resulting digital data stream is then re-modulated onto a second RF signal at the repeater station. The re-modulation uses a second modulation scheme having a higher-order modulation as compared to the first modulation scheme. Following re-modulation the second RF signal having the higher-order modulation scheme is then transmitted from the repeater station over the backhaul communication link to the base station.
In accordance with the invention, each of the first and the second RF signal is comprised of a plurality of TDM channels and each mobile unit is assigned one of the TDM channels of the first RF signal. Furthermore, a plurality of the TDM channels of the first RF signal are compressed into a single TDM channel on the second RF signal. Additionally, the first modulation scheme has a first associated data rate and the second modulation scheme has a second associated data rate, the second data rate being at least equal to the first data rate. Moreover, the second data rate is at least equal to the combined data rate of the TDM channels comprising the first RF signal.
In another aspect of the invention, the first modulation scheme is Gaussian Minimum Shift Keying (GMSK). In another aspect of the invention, the second modulation scheme is a Phase Shift Keying (PSK) scheme having at least 8 states. It should readily be understood by one skilled in the art that any other high order modulation scheme can be utilized without departing from the spirit of the invention. For example, a Quadrature Amplitude Modulation (QAM) scheme having at least 16 states (16-QAM) could be used.
In yet a further embodiment of the invention, a method is provided for aggregating data on a backhaul communication link in a wireless communication system having a base station located within a home cell, and a plurality of substantially adjacent cells each having a repeater station located therein. The method comprises, receiving at a repeater station, uplink RF carrier signals from each of a plurality of mobile units at a repeater station. Each of the received uplink RF carrier signals from the plurality of mobile units are then compressed at the repeater station into a single RF carrier signal. The single RF carrier signal containing the compressed uplink RF carrier signals is then transmitted from the repeater station on an uplink channel of a backhaul communication link to a serving base station within a home cell.
In yet another embodiment of the instant invention, a system for supporting increased capacity on a backhaul communication link carrier frequency in a wireless communication system having a base station located within a home cell, and a plurality of substantially adjacent cells each having a repeater station located therein, is disclosed. The system is comprised of a receiver for receiving at one of the repeater stations, a first RF signal having a first modulation scheme from a mobile transceiver unit on an uplink channel. A demodulator is provided within the receiver for demodulating at the repeater station, the first RF signal from the mobile transceiver unit to obtain a digital data stream. A digital communication link communicates the digital data stream from the demodulator to a modulator. The modulator modulates the digital data stream onto a second RF signal at the repeater station using a second modulation scheme having a higher-order modulation as compared to the first modulation scheme. A transmitter coupled to the modulator transmits from the repeater station, the second RF signal having a higher-order modulation over the backhaul communication link to the base station.
In accordance with the invention, each of the first and the second RF signal is comprised of a plurality of TDM channels and each mobile unit is assigned one of the TDM channels of the first RF signal. Furthermore, a plurality of the TDM channels of the first RF signal are compressed within a single TDM channel on the second RF signal.
Furthermore, the first modulation scheme has a first associated data rate and the second modulation scheme has a second associated data rate. Additionally, the second data rate is at least equal to the combined data rate of the TDM channels comprising the first RF signal.
In one aspect of the invention, the first modulation scheme is Gaussian Minimum Shift Keying (GMSK). In another aspect of the invention, the second modulation scheme is a Phase Shift Keying (PSK) scheme having at least 8 states (8-PSK). It should be readily understood by one skilled in the art that any higher-order modulation scheme can be used without departing from the spirit of the invention. For example, a Quadrature Amplitude Modulation (QAM) scheme having at least 16 states (16-QAM) can be used.
In still a further embodiment of the instant invention, a system for aggregating data on a backhaul communication link in a wireless communication system having a base station located within a home cell, and a plurality of substantially adjacent cells each having a repeater station located therein, is disclosed. The system comprises a receiver for receiving uplink RF carrier signals from each of a plurality of mobile units at a repeater station. A signal processor coupled to the receiver is used for compressing at the repeater station, each of the received uplink RF carrier signals from the plurality of mobile units into a single RF carrier signal. A transmitter coupled to the signal processor is used for transmitting the single RF carrier signal containing the compressed uplink RF carrier signals from the repeater station, on an uplink channel backhaul communication link, to the serving base station within the home cell.